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Simple Pendulum — Time period, effective length

SHM

8

JEE Qs

8%

Hard

75

min

Always correctly identify the effective length 'L' and the effective gravitational acceleration 'g_eff' for any given scenario to solve pendulum problems accurately.

🧮 Key Formulas

For small angular displacements (θ): T = 2π * sqrt(L/g)
Angular frequency: ω = sqrt(g/L)
Restoring force (tangential): F_t = -mg sinθ ≈ -mgθ (for small θ)
Effective gravity (g_eff) in accelerating frame: e.g., for upward acceleration 'a', g_eff = g + a; for downward 'a', g_eff = g - a; for horizontal 'a', g_eff = sqrt(g^2 + a^2) and pendulum aligns with g_eff

✅ Key Points for JEE

  • 1The simple pendulum exhibits SHM only for small angular displacements (typically less than 10-15 degrees), where sinθ ≈ θ.
  • 2The 'L' in the formula T = 2π * sqrt(L/g) represents the effective length, which is the distance from the point of suspension to the center of mass of the bob.
  • 3The time period of a simple pendulum is independent of the mass of the bob and the amplitude (for small angles).
  • 4In non-inertial frames (e.g., accelerating elevator), the value of 'g' must be replaced by the effective gravitational acceleration (g_eff) acting on the pendulum.
  • 5When a pendulum is immersed in a fluid, the buoyant force affects its motion. 'g' in the formula should be replaced by g * (1 - ρ_fluid/ρ_bob), and the effective length might change if the string or bob's properties are affected.

⚠️ Common Mistakes

  • Using the simple pendulum time period formula for large amplitudes, where the motion is not truly SHM and the formula is inaccurate.
  • Incorrectly identifying the effective length 'L', especially when dealing with pendulums that are not ideal point masses or have unusual suspension.
  • Failing to correctly calculate or substitute the effective gravitational acceleration (g_eff) in accelerating frames or when external forces like buoyancy are present.
  • Confusing the simple pendulum with a physical pendulum, which has a different formula for its time period.

📝 Practice Questions

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Q36.A particle oscillates along the x-axis according to the law, x(t) = x0 sin2 ( 2t ) where x0 = 1 m . The kinetic energy (K) of the particle as a function of x is correctly represented by the graph (1) (2) (3) (4)

2025·Graph basedMedium

Q28.Two bodies A and B of equal mass are suspended from two massless springs of spring constant k1 and k2 , respectively. If the bodies oscillate vertically such that their amplitudes are equal, the ratio of the maximum velocity of A to the maximum velocity of B is (1) k1 (2) k2 √k1k2 (3) (4) k2 k1 √k2k1

2025·MCQEasy

Q41.A light hollow cube of side length 10 cm and mass 10 g , is floating in water. It is pushed down and released to execute simple harmonic oscillations. The time period of oscillations is yπ × 10−2 s, where the value of y is (Acceleration due to gravity, g = 10 m/s2 , density of water = 103 kg/m3 ) 2025 (23 Jan Shift 1) JEE Main Previous Year Paper (1) 6 (2) 2 (3) 4 (4) 1

2025·NumericalMedium

Q36.A particle is executing simple harmonic motion with time period 2 s and amplitude 1 cm . If D and d are the total distance and displacement covered by the particle in 12.5 s , then D is d (1) 16 (2) 10 5 (3) 15 (4) 25 4

2025·MCQMedium

Q35.Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : Knowing initial position x0 and initial momentum p0 is enough to determine the position and momentum at any time t for a simple harmonic motion with a given angular frequency ω. Reason (R): The amplitude and phase can be expressed in terms of x0 and p0 . In the light of the above statements, choose the correct answer from the options given below : (1) (A) is false but (R) is true (2) (A) is true but (R) is false (3) Both (A) and (R) are true but (R) is NOT the (4) Both (A) and (R) are true and (R) is the correct correct explanation of (A) explanation of (A)

2025·Assertion ReasoningMedium

Q24.The displacement of a particle executing SHM is given by x = 10 sin (wt + π3 )m. The time period of motion is 3.14 s. The velocity of the particle at t = 0 is ______ m/s.

2024·NumericalEasy

NCERT Chapters

  • Class 11 Physics Ch 14: Oscillations